Modern, computer-assisted surgery is able, during an operation, to display a surgeon's instruments or implants on a monitor in relation to anatomical data obtained beforehand from a patient scan (e.g., CT or MR scans). To this end, the instruments and/or implants have to be calibrated. For example, the spatial position and/or the position of the functional section, for example, the tip of an instrument, in the medical navigation system being used has to be known. One option is to perform pre-calibration (pre-operative calibration), i.e., disclosing the geometric data of the instrument or implant to the navigation system in advance and storing the data securely on the software side. Another option is so-called intra-operative calibration, in which the instrument or implant is calibrated during the operation by the staff carrying out the treatment.
Using pre-operatively calibrated instruments or implants nonetheless makes it necessary in many cases to intra-operatively verify the accuracy of the instruments and re-calibrate them. In contrast, the use of intra-operative calibration makes it possible to fall back on available instruments, without being reliant on those instruments whose geometry has already been stored on the software side. In other words, each surgeon can, for example, use his own instruments. Intra-operative calibration is advantageous when instruments have changed between two operations (e.g., by re-sharpening an instrument) but have to be highly accurately calibrated during the operation.
Intra-operative calibration be performed in various ways. In one conventional calibration method, only so-called point calibration is performed, where only the length of the instrument or implant is determined, and not its geometry. A second and continuative conventional method involves determining, alongside the length, the exact vector of the instrument, i.e., its geometry, as well. Various methods and aids are used in this respect. However, each of these methods and aids are currently limited to rotationally symmetrical instruments, which are very easy to calibrate. With regard to such techniques, reference is made to U.S. Pat. No. 6,021,343, WO 96/11624 and U.S. Pat. No. 5,921,992. With these techniques, in order for the instrument to be calibrated, it must be clamped or otherwise inserted, either during the operation or shortly before the instrument is used, into a fixed calibration tool, i.e., into a means that is fixed positionally and with respect to the patient.
The disadvantage of the system described in U.S. Pat. No. 5,921,992 is again that calibration is only performed with respect to the orientation of the instrument and its punctiform tip. The system is limited in that only instruments that are suitable to being calibrated are those in which the position of the tip of the instrument and the subsequently linearly running section are important to the treatment.